This invention relates to tethered vehicles such as optical fiber guided missiles, and, more particularly, to the determination of the absolute location of such tethered vehicles.
Tethered vehicles are used in a variety of civilian and military missions. Such a tethered vehicle typically includes a self-propelled, unmanned vehicle that is linked to a central control station by a wire or optical fiber data link. Information is transmitted from the vehicle to a controller along the data link, and control signals are transmitted from the controller to the vehicle along the same data link. Examples of such tethered vehicles include missiles, boats, torpedoes, certain spacecraft, and explorer and salvage units. Optical fiber guided missiles are of the most interest to the present inventors, and will be discussed in greatest detail herein, but the present approach is applicable to other types of tethered vehicles as well.
An optical fiber guided missile system includes a missile, a control station, and an optical fiber data link extending between the missile and the control station. The missile is usually launched from the vicinity of the control station, which may be a fixed or mobile ground site or an aircraft. The optical fiber is initially wound onto a bobbin in the missile (or one bobbin in the missile and another at the launch site) and payed out from the missile as the missile flies. Optical fibers used in such missile systems are typically 5-30 kilometers in length or even longer in some cases, defining the radius of operation of the missile from its launch site. Optical fiber guidance has the important advantage over other types of guidance systems that it is highly resistant to jamming and other interference, and can bidirectionally transmit large quantities of information simultaneously from and to the missile.
As the missile flies through the air, a sensor such as a visible-light television camera or an infrared seeker produces a picture of the terrain. The picture is transmitted back to the control station on the optical fiber data link, where the operator or an electronic tracker uses the picture in selecting targets, performing reconnaissance, or other missions. Control signals are transmitted back along the optical fiber to the missile from the control station, responsive to the commands of the operator or tracker.
For many missions the absolute position of the missile must be known, particularly where the radius of operation takes the missile to great distances from the control station. In one type of mission, for example, the missile may initially fly at low speeds at various altitudes and headings to gather reconnaissance data and then, after identifying the target, switch to a higher speed attack at a previously defined location. When flying such a mission profile in the confusion of the battlefield environment, the operator or tracker may lose track of absolute position of the missile with respect to the control station, interfering with the targeting procedure and reducing the value of the data gathered during the reconnaissance phase.
It is therefore important to be able to determine the position of the optical fiber guided missile. Visual and radar methods cannot be relied upon, because the missile may be outside the line of sight and because the radar returns may be unavailable or unreliable when the missile is flying at a low altitude. Relative position of the missile calculated from heading and speed information may provide an approximation of the absolute position data, but there is always a substantial degree of uncertainty of the missile position computed in this way.
Another possible solution is to use the global positioning system (GPS) to determine the absolute position of the missile. GPS provides an array of satellites that transmit positioning signals. The position of a receiver of those signals can be determined by a ranging method, wherein the position is uniquely determined by the range of the receiver to three, four, or more satellite transmitters.
The use of GPS in an optical fiber guided missile is made complex by the need to establish the position of the missile very accurately and very rapidly, while working within the missile constraints of low weight and acceptable cost. A variety of GPS receivers are available. The faster, more accurate GPS receivers tend to be heavy and costly, while the lighter, less costly GPS receivers cannot make position determinations rapidly enough to be of tactical value. Many existing GPS signal processing units also cannot stand the demanding operational environments experienced by a missile.
There remains a need for a tethered vehicle positioning system operable with an optical fiber guided tethered vehicle. The present invention fulfills this need, and further provides related advantages.